Gantry-type cranes are often outfitted to serve as container cranes for loading standard freight containers into container ships, and also for unloading the containers from the ships. Typically, a container ship has a large number of cells or compartments in which standard freight containers can be received with only a minimum of clearance, and can be stacked vertically until the cells are full. In order to lower a freight container into a cell, the container must be positioned with a high degree of accuracy over the cell so that the container can be lowered directly into the cell without bumping the deck of the ship or the walls of the cell to any objectionable extent. A gantry-type container crane comprises a substantially horizontal supporting structure or boom with rails thereon along which a trolley is moveable in either direction by an electrically controlled power drive. A hoisting means or system is suspended from the trolley and is moveable horizontally therewith. The hoisting system comprises a system of wire ropes hanging downwardly from the trolley and connected to a load carrying device, preferably a spreader bar grasping device for selectively grasping and releasing a freight container.
A container crane is well adapted for unloading containers from railroad cars or semi-trailer trucks and for loading the containers into the cells of a container ship. In a typical sequence of operations, the trolley is moved horizontally along the boom and is stopped directly over a container on a waiting semi-trailer truck. The spreader bar is lowered by the hoisting system into engagement with the container and is actuated so as to grasp the container, which is then hoisted to a safe elevation so that the container will clear any obstacles on the dock or the container ship. The trolley is then moved outwardly along the boom and over the container ship until the trolley is over the cell into which the container is to be loaded. The object of this maneuvering is to enable the container to be lowered by the hoisting system directly into the cell.
However, considerable difficulty has been experienced by crane operators in aligning the container with the cell with sufficient accuracy to enable the container to be lowered into the cell without any objectionable bumping of the container against the deck of the ship or the walls of the cell. This difficulty arises from the fact that the container starts to swing like the bob of a pendulum when the trolley is stopped. The container may swing through several pendulum cycles before the swinging movement is dissipated sufficiently to enable the crane operator to lower the container into the cell. The trolley constitutes the pivotal support for the pendulum. The suspension means are formed by the hoisting rope system, and the bob is formed by the container and the spreader bar. The problem arising from the pendulum swinging of the container has been widely recognized, but no satisfactory solution has heretofore been devised.